1. Field of the Invention
This invention is related in general to the field of acoustic detection systems. In particular, the invention consists of a device for detecting excessive noise that exceeds a threshold for a specified period of time or is repetitive.
2. Description of the Prior Art
Noise detection systems have been used in hospitals, offices, or other environments where excessive noise levels are of concern. A simple noise detection system is based on using a sound level meter or dosimeter to detect when acoustic noise exceeds a cut-off level. The force of sound striking a pressure transducer creates an electrical signal whose amplitude and component frequencies are analogous to the sound's pressure variations. This electrical signal is then integrated by the dosimeter over various frequency ranges to obtain a corresponding noise level. These noise levels are then integrated to produce a signal representative of the power of the sound striking the pressure transducer. The resulting power signal is multiplied by the period of time of the duration of the sound striking the pressure transducer to provide a signal representative of the energy contained in the sound. This energy signal, a voltage signal representative of a corresponding decibel level, is compared to a pre-determined cut-off level. If the energy signal meets or exceeds the cut-off level, an alarm may be triggered. A system such as this may be used in a hospital to alert staff that excessive noise may be bothersome or dangerous to some of the patients. The alert can be in the form of a remote speaker or visual display such as a flashing light.
The problem with dosimeters or sound level meters is that they are prone to false triggering events. For example, a bell-like sound or ding in a hospital room may be of sufficient energy to trigger an alarm notification, even if the noise is transient in nature, is required to ensure proper care, or is not likely to disturb a patient. If a traditional noise detection system is used, alerts or alarms would occur with such frequency that they would be eventually ignored by the staff or the alerting system would be disconnected.
Another problem with current sound detection devices is that they do not effectively indicate when a bothersome noise occurring below the cut-off level occurs in a repetitive manner. For example, persons talking, persons clapping their hands or common construction noises such as hammering typically include high energy impulse sound interspersed with periods of relative quiet. An integration of this noise will produce an energy level much lower than that occurring during the sound impulses. In order for current sound detection systems to detect these impulses, the sampling frequency will need to be relatively high.
Additionally, current sound detection systems utilizing dosimeters are inadequate for separating objectionable noise from non-objectionable noise. Noise emanating from air-conditioning units or HVAC ducts is typically low-frequency, steady-state sound that is usually not considered objectionable. However, the double integration of signals coming from pressure transducers does not, by itself, separate high-frequency noise from low-frequency noise. Additionally, some types of noise are unavoidable and triggering an alert when they happen may be counter-productive, such as a single transient event (e.g., dropping a tray) or the sound created by ventilators in a patient's room.
Accordingly, it would be advantageous to have a system that detects high-energy repetitive noise and moderate energy steady-state noise. Additionally, it would be advantageous to have a system that minimizes false triggering. It would also be desirable to have a system that gives different weight to low-frequency noise than it does for high-frequency noise. Yet another desirable feature of a sound-detection system is one that can be configured to accommodate unavoidable noises.